The present invention relates in general to methods of manufacturing a solar cell.
In a conventional method of making a solar cell, particularly making a p-n junction in a p type silicon substrate, the p-n junction is formed by diffusing phosphorus into a surface region of the silicon substrate by a thermal diffusion method. To form the p-n junction, a heat treatment of more than 850.degree. is required. However, such a heat treatment causes heat damage to the silicon substrate. This results in a significant decrease in the lifetime of minority carriers, which is a very important parameter of a silicon semiconductor relating to its operating efficiency. With a significant decrease in minority carrier lifetime, a semiconductor device of high performance cannot be obtained.
Recently, there has been active research in the ion implantation method, which is one technique for forming a semiconductor junction without the use of thermal diffusion. In the ion implantation method, the degradation of minority carrier lifetime is less than the degradation associated with the heat diffusion method. However, the ion implantation approach has a different drawback. The crystallographic structure of an ion implanted layer is disturbed and the implanted ion is not adequately activated. This prevents obtaining satisfactory characteristics of the semiconductor device produced with the ion implantation method. The semiconductor substrate must be annealed to overcome this deficiency associated with ion implantation.
There are two annealing processes: one is furnace annealing and the other is laser annealing. The ion implantation method cannot provide a high quality semiconductor device without utilizing either the furnace annealing process alone or a combination of both furnace and laser annealing. Because of the annealing requirement associated with ion implantation, minority carrier lifetime is decreased undesirably. At the present state of the art, a combination of ion implantation and laser annealing is not available to produce solar cells. Further, ion implantation requires a relatively large apparatus and the amount of semiconductor substrate that can be treated within a predetermined period of time is relatively small for solar cell production. This would make such an approach cost inefficient.
Therefore, it is highly desirable in the solar cell industry to provide a method of making a semiconductor junction that would not significantly decrease minority carrier lifetime, while at the same time permit the production of a low cost semiconductor device.